1. Field of the Invention
The present invention relates to the field of optically pumped atomic clocks, and more particularly to a method and system including a laser that is self-modulated by alkali-metal vapor at 0-0 atomic-clock frequency by using light of alternating polarization, referred to as push-pull optical pumping technique, and uses electrical modulation across the laser diode as a clock signal.
2. Description of the Related Art
Gas-cell atomic clocks and magnetometers use optically pumped alkali-metal vapors. Atomic clocks are applied in various systems that require extremely accurate frequency measurements. Atomic magnetometers are utilized in magnetic field detection with extremely high sensitivity. For example, atomic clocks are used in GPS (global positioning system) satellites and other navigation systems, as well as in high-speed digital communication systems, scientific experiments, and military applications. Magnetometers are used in medical systems, scientific experiments, industry and military applications.
A vapor cell used in atomic clocks or magnetometers contains a few droplets of alkali metal, such as potassium, rubidium, or cesium. A buffer gas, such as nitrogen, other noble gases, or a mixture thereof, is required to be filled inside the cell to match the spectral profile of the pumping light, suppress the radiation trapping, and diminish alkali-metal atoms diffusing to the cell wall. The gas cell is heated up to above room temperature to produce sufficient alkali-metal vapor. The resonances of alkali-metal ground-state hyperfine sublevels are especially useful for atomic clocks and atomic magnetometers. The hyperfine resonance is excited by rf (radio frequency) fields, microwave fields, or modulated light (CPT: coherent population trapping method). The resonance is probed by the laser beam. As shown in FIG. 1, hyperfine 0-0 resonance, ν00, is particularly interesting for atomic clocks because of its insensitivity of the magnetic field at low field regime; hyperfine end resonance, νend, can be used either for atomic clocks and magnetometers; the Zeeman end resonance, νZ, is usually used for a magnetometer because of its high sensitivity of the magnetic field. Besides the three illustrative resonances, other resonances of different hyperfine sublevels can also be used for atomic clocks and magnetometers. The resonance signal is reflected on the probing beam as a transmission dip or a transmission peak when the frequency is scanned through the resonance frequency. Conventionally, an atomic clock or a magnetometer measures the frequency at the maximum response of the atomic resonance. A local oscillator is required to generate the oscillation signal and excite the resonance. A precise clock ticking signal is therefore provided by the output of the local oscillator.
U.S. Pat. No. 7,323,941, hereby incorporated by reference in its entirety into this application, describes a self-modulated laser system 10, as shown in FIG. 2. No local oscillator is needed. Self-modulated laser system 10 uses polarization gain medium 12, such as an electronically pumped semiconductor, for example, quantum well heterojunction edge-emitting laser diode (ELD). Polarization gain medium 12 outputs light with linear polarization. In order to generate the alternation of photon spin, two quarter wave plates 13a, 13b are used inside laser cavity 11. Vapor cell 14 is positioned, where the laser beam has the maximum alternation of the light polarization, between quarter wave plates 13a, 13b. Bragg mirror 15 and output coupler 16 recombine beams so that they emerge as a single beam of alternating circular polarization. The transmission of light through external cavity 11 is measured with photodiode 18 to generate clock signal 19.
It is desirable to provide an improved method and system for reducing complexity, size and power consumption of an atomic clock.